1. Field of Invention
The present invention relates to the polishing head of a chemical-mechanical polishing (CMP) machine. More particularly, the present invention relates to an improved retainer ring design for the polishing head.
2. Description of Related Art
Chemical-mechanical polishing is a global planarization technique providing planar surfaces for very large scale integration (VLSI) or even ultra-large scale integration (ULSI) circuits. As shown in FIGS. 1A and 1B, a conventional chemical-mechanical polishing machine has a polishing table 10 and a polishing pad 12 above the polishing table. The chemical-mechanical polishing machine also has a polishing head. Inside a polishing head 14, a wafer 16 whose top surface is facing down is loaded between the polishing head 14 and the polishing pad 12. Slurry 18 is carried to the polishing pad surface through a supply pipeline.
FIG. 1C is a detailed cross-sectional view of the polishing head 14. As shown in FIG. 1C, at the upper end of the polishing head 14 there is an air chamber 20 for exerting pressure on a wafer loader 22 below. At the periphery below the wafer loader 22, there is a wafer retainer ring 24 for retaining a wafer 16 within. In addition, a buried pad (not shown in the Figure) is usually installed between the wafer loader 22 and the wafer 16.
FIGS. 2A and 2B show the respective top view and a bottom view of a conventional wafer retainer ring 24. As shown in FIGS. 2A and 2B, the retainer ring 24 is a smooth circular structure having an outer diameter of about 25.4 cm and inner diameter of about 22.86 cm. Due to a lack of conduit structures in the retainer ring 14 for allowing the slurry to enter the polishing head, slurry is difficult to get to the central portion during polishing operation. Furthermore, the cycling of slurry is spasmodic, and hence resulting in an unevenly polished wafer surface. In other words, polishing defects such as non-uniformity, big wafer edge exclusion, slow polish-removal rate, ineffective use of slurry, and a reduced lifetime for polishing pad and buried pad will arise.
In light of the foregoing, there is a need to provide an improved wafer retainer ring design for the polishing head.